Mobile phones, notebook PCs, video cameras, digital cameras, and other electronic equipment use a surface-mountable electrochemical device, such as an electric double-layer capacitor or lithium ion battery, as their power source suitable for backing up the memory and the like.
This electrochemical device generally has: an insulating case with a concaved section constituting an opening in the top surface; a conductive lid that closes the concaved section of the case in a water-tight and air-tight manner; a chargeable/dischargeable electric storage element and electrolyte enclosed in the closed concaved section; a positive electrode terminal and negative electrode terminal provided on the mounting surface of the case; a positive electrode wiring for electrically connecting the positive electrode terminal and the positive electrode side of the electric storage element; and a negative electrode wiring for electrically connecting the negative electrode terminal and the negative electrode side of the electric storage element (refer to Patent Literature 1).
The electric storage element is constituted by a first electrode sheet of specified size made of active material, a second electrode sheet of specified size made of active material, and a separator sheet of specified size made of ion-permeating sheet, which are stacked together in the order of the first electrode sheet, separator sheet, and second electrode sheet. The outer periphery part of the separator sheet whose external dimensions are slightly larger than the external dimensions of the two electrode sheets extends slightly outward from the two electrode sheets. The material of the first electrode sheet may be the same as or different from the material of the second electrode sheet depending on the type of the electrochemical device.
Additionally, for the separator sheet whose functions include preventing the first electrode sheet and second electrode sheet from shorting with each other, retaining the electrolyte between the facing surfaces of the first electrode sheet and second electrode sheet, and allowing the ions to move in the retained electrolyte, a fiber-based porous sheet is generally used whose thickness is suitable for achieving these functions. For example, Patent Literature 2 describes a separator for electric double-layer capacitor constituted by a porous sheet and having a high-density layer with a void ratio of approx. 20 to 50% and low-density layer with a void ratio of approx. 50 to 80%.
With the aforementioned electrochemical device, the electrolyte is mostly impregnated into the first electrode sheet, second electrode sheet, and separator sheet and does not flow much in the charge/discharge process, but if the electrolyte in the two electrode sheets breaks down, deteriorates, or undergoes other change during this process, then the electrolyte impregnated into the part of the separator sheet sandwiched between the two electrode sheets may be drawn into the two electrode sheets, thus causing a phenomenon of the electrolyte in this part decreasing, albeit by a very small amount.
If this phenomenon occurs, the part of the separator sheet sandwiched between the two electrode sheets tries to draw in the amount of electrolyte corresponding to what has been drawn into the two electrode sheets, from the part of the separator sheet extending outward from the two electrode sheets. However, since the part of the separator sheet extending outward from the two electrode sheets has the same thickness and liquid absorptivity as the part sandwiched between the electrode sheets, it is difficult to instantly draw the electrolyte into the part sandwiched between the two electrode sheets from the part extending outward from the two electrode sheets. Also because the amount of electrolyte impregnated into the part extending outward from the two electrode sheets is very small, frequent occurrences of the aforementioned phenomenon will not prevent any decrease in the amount of electrolyte in the part of the separator sheet sandwiched between the two electrode sheets and, as a result of accumulation of this phenomenon, the charge/discharge characteristics will drop.
Note that the aforementioned term “liquid absorptivity” corresponds to the water absorption rate as measured by the Byreck method specified in JIS-L-1907. Also in the “Modes for Carrying Out the Invention” section of the Specification, mm/10 min is used as the unit of liquid absorptivity.